In the manufacture of aluminum and aluminum alloy containers, container manufacturers purchase long rolls of sheet aluminum from aluminum companies as the raw material for the manufacture of the containers. In order to prevent abrasion of the surfaces of these aluminum sheets, so-called "rolling oils" are applied to the surfaces thereof prior to rolling the sheets for shipment to the container manufacturers. The container manufacturers then use the sheet aluminum in drawing and forming operations, usually referred to as "drawing and ironing" to form the aluminum containers. In these drawing and ironing operations lubricants and forming oils are used on the surfaces of the aluminum to facilitate these operations.
In order to remove the rolling oils, lubricants and forming oils, as well as aluminum fines, all of which are usually present on the inner and outer surfaces of the aluminum containers, cleaning operations must be employed prior to further processing of the containers. Present commercial container lines typically utilize the following basic steps: prewash.fwdarw.acid cleaning.fwdarw.tap water rinse.fwdarw.conversion coating.fwdarw.tap water rinse.fwdarw.deionized water rinse. Following this cleaning and surface treatment sequence, the containers are dried and are then suitable for further processing, including the application of siccative coatings and labeling inks.
One of the largest uses of these aluminum containers is for liquid comestibles, such as beer, soft drinks, etc. During the past five years or so, occasional problems have arisen with off-flavor and off-taste, particularly with beer. This problem appears to be aggravated by periods of storage of the filled containers prior to consumption. After intensive study of the problem by brewers, can manufacturers, consulting firms, and manufacturers of aluminum cleaning products, it was determined that the problem was caused, at least in part, by the presence of minute quantities of C.sub.7, C.sub.8 and/or C.sub.9 unsaturated aldehydes in the beer. Such aldehydes are particularly strong flavorants. For example, 2-nonenal is detectible in beer at a level of one part per billion (ppb) and is objectionable at a level of 2 parts per billion. These aldehydes are formed from the oxidation of unsaturated fatty acids such as oleic, linoleic, and linolenic acid, one or more of which unsaturated fatty acids are usually present in the rolling oils, lubricants, and/or forming oils used in processing the aluminum containers. A good discussion of this problem is given in an article entitled "Two-Piece Cans: Some Flavor Problems Caused by Manufacturing Materials or Practices", by William A. Hardwick, MBAA Technical Quarterly, Vol. 15, No. 1, 1978.
Attempts to eliminate this taste problem (commonly referred to as the "labox" problem) in beer have not to date been uniformly successful. For example, aluminum container manufacturers have not been successful in totally eliminating the presence of unsaturated fatty acids from their lubricants and forming oils. Also, efforts by manufacturers of aluminum cleaning products have not heretofore been able to devise a cleaning system that results in complete removal of unsaturated fatty acids from the surfaces of the aluminum containers and the labox problem has not been solved to date in any consistently satisfactory manner.